Dyeing by traditional water-based methods and subsequent washing processes produces large amounts of, usually strongly coloured, waste water. Furthermore, when dyeing, for instance, polyester fibers from an aqueous medium, the dyed fibers need to be subjected to a so called reduction clearing which causes additional effluent problems.
The aforementioned environmental drawbacks of water-based dyeing methods can be overcome by dyeing from supercritical carbon dioxide. Supercritical dyeing additionally offers the advantage that densities and viscosities in supercritical carbon dioxide are lower and diffusion more rapid than in liquids, shortening the process time.
The dyeing of substrate materials in liquid or supercritical carbon dioxide is well-known in the art. It is also known to employ reactive dyeing substances in supercritical dyeing methods that are capable of reacting with the substrate under the formation of a chemical bond. These reactive substances are usually derivatives of CO2-soluble disperse dyestuffs (chromophores) that contain a reactive group that is capable of reacting with specific residues in the substrate.
U.S. Pat. No. 6,620,211 describes a method for dyeing textile material with one or more fibre-reactive disperse dyestuffs in a supercritical or almost supercritical fluid, wherein the relative humidity of the fluid is in the range of 10-100% during dyeing. It is observed in the patent that maintaining the relative humidity of the fluid in the range from 10 to 100% during the dyeing process ensures that the textile material remains sufficiently moist and therefore sufficiently accessible for the uptake of the dyestuff. In the patent it is observed that the moistening agent may contain a reaction-accelerating auxiliary for accelerating the reaction between the reactive disperse dyestuff and the textile material. Examples of these auxiliaries are said to include pyridine and ammonium salts. It is said that these reaction accelerators often contain tertiary and quaternary ammonium groups.
U.S. Pat. No. 6,010,542 describes a method of dyeing a substrate in carbon dioxide comprising the steps of (a) providing a dye composition comprising carbon dioxide, a dye and a surfactant, the surfactant being included in an amount sufficient to solubilise, emulsify or disperse the dye in the carbon dioxide, and then (b) dyeing said substrate with said dye composition. According to the US patent the method enables the use of relatively large and less volatile dyes in carbon dioxide mediated dyeing systems. Furthermore, it is said that the method does not require high temperatures and pressures for sufficient dissolution of the dye. The patent provides a long list of additives that may be employed with the carbon dioxide, including carboxylic acids.
The results obtained with supercritical dyeing methods described in the prior art are not entirely satisfactory. In particular, the colour yields and wash-fastening properties of the dyed substrate are often disappointing.